The big reason to avoid circle top windows or watch out for this phase of construction.

I have seen this literally a hundred times.  No one puts insulation in the quarter circles.  Around the rectangles they do, but not that big arched triangle.  No wonder it's cold around the circletops.

This is a remodel from a late 60's build.  The sheetrock was removed to show a fairly large void with a two by four block to hold it at the right thickness.  No insulation at all, foam, expanding, non-expanded or otherwise.  This is quite typical in every home I work in with arches and half circles.

Happy Window Research!